"We suffer from this illusion, or reality I suppose, where some problems become so big no one wants to get involved.

"She was the one who wanted to change that.

Princess Diana at a ceremony at Red Cross HQ back in 1997 (Image: AFP)

"I will always look up to her as my ideal role model. Everything she did was having an impact, it was making a difference."

He added that he believed Diana had "rewritten the rule book" for the royals, and the role they play in society.

He said: "All the people she was working with would be incredibly grateful for her pushing those boundaries."

The Duke of Cambridge and Prince Harry leave following a visit to the White Garden in Kensington Palace (Image: PA)

Earlier in the day, he also said he believes young people hold "the solutions to some of the world's biggest problems".

The Prince, who attended the event a year on from when news of his relationship with American actress Meghan Markle hit the headlines, said the younger generation had "turned me into an optimist", as he talked about meeting young people across the world.